1. Field of the Invention
The present invention relates to a communication apparatus, a communication system and a network establishing method, and particularly relates to a technique for switching communication standards applied to a network depending on the compatibility of communication apparatuses with communication standards.
2. Description of the Related Art
With a wireless LAN, an infrastructure mode and an ad hoc mode are known as communication modes by which a plurality of communication apparatuses communicate. With the infrastructure mode, communication apparatuses communicate with one another via a base station (access point). With the ad hoc mode, on the other hand, communication apparatuses communicate directly rather than via an access point.
With the ad hoc mode, communication apparatuses transmit a broadcast signal called a beacon on a rotational basis since there is no access point. In order for the communication apparatuses to maintain the network, each beacon contains network synchronization information and transmission rate information in addition to network identification information (SSID) and the MAC address of the source. Beacons are broadcast. To connect to a network in the ad hoc mode, a communication apparatus conducts a network search. The communication apparatus conducts this search by transmitting a probe request as a search signal.
Another communication apparatus, having received a beacon, sends back a probe response as a response signal. A given communication apparatus, when unable to receive a probe response despite having transmitted a fixed number of probe requests, judges that a network for connecting to does not exist, and establishes a network by starting to transmit beacons.
When able to receive a probe signal, the communication apparatus joins the network to which the communication apparatus that sent the probe response belongs.
The above wireless LAN standards are contained in the following document.
IEEE STD 802.11-1999 Part 11, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.